Storage system for radioactive nuclear waste with pressure surge protection

ABSTRACT

A radioactive nuclear waste storage system includes a cask comprising a hermetically sealed internal cavity configured for holding the waste such as spent nuclear fuel submerged in an inventory of water. One or more pressure surge capacitors disposed inside the cask include a vacuum cavity evacuated to sub-atmospheric conditions prior to storage of fuel in the cask. At least one rupture disk seals a vacuum chamber inside each capacitor. Each rupture disk is designed and constructed to burst at a predetermined burst pressure level occurring inside the cask external to the capacitor. This allows excess cask pressure occurring during a high pressure excursion resulting from abnormal operating conditions to bleed into capacitor, thereby returning the pressure inside the cask to acceptable levels. In one embodiment, the capacitors are located in peripheral regions of the cask cavity adjacent to the circumferential wall of the cask body.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/220,560 filed Apr. 1, 2021, which claims the benefit of U.S.Provisional Application No. 63/003,431 filed Apr. 1, 2020; which are allincorporated herein by reference in their entireties.

BACKGROUND

The present invention relates generally to systems and vessels forstoring and transporting high level radioactive nuclear waste such asused or spent nuclear fuel (SNF), and more particularly to a wet caskstorage system with overpressurization protection.

A typical module or cask employed to store and transport fissileradioactive waste such as SNF uses an inert gas such as helium toprotect the elongated zirconia metal tubes (also referred to as tubecladding) of the fuel rods from oxidation. Such casks insert gas filledcasks are referred to as a “dry cask.” Multiple fuel rods are bundledtogether in a support structure referred to as a fuel assembly which arewell known in the art without undue elaboration here. The fuelassemblies are liftable structures typically having a rectangular cuboidshape for U.S. reactors which are configured for insertion into thereactor as self-supporting units.

A gas filled dry cask (“dry cask”) holding multiple such fuel assemblyunits, however, is not perfect from the standpoint of controlling thetemperature of the heat-emitting nuclear waste fuel because the heatrejection rate from such a vessel is inhibited by the low thermalconductivity of the gaseous media. This exposes the zirconia fuel rodtubes (cladding) to oxidation and damage, thereby adversely affectingthe structural integrity of the containment provided by the tubes forthe fissionable nuclear fuel material (e.g. uranium ceramic pellets)packed inside the fuel rods.

Improvements in cask storage systems for radioactive nuclear waste isdesired.

BRIEF SUMMARY

The present application discloses a wet cask system for storing andtransporting radioactive nuclear waste such as without limitation spentnuclear fuel (SNF). The system includes an unventilated and hermeticallysealed cask containing a volume or inventory of water in which the SNF(e.g. fuel assemblies) is submerged. In one embodiment, the water may beborated for additional radiation shielding. The body of the caskcomprises radiation shielding to block and attenuate gamma and neutronradiation emitted by the SNF assemblies.

The wet cask system preferably further includes a pressure controlsub-system for limiting internal pressure in the cask during a highpressure excursion conditions. In various embodiments, the pressurecontrol sub-system comprises one or more evacuated internal pressurecontrol devices which may be pressure surge capacitors in oneembodiment. These capacitors are configured for insertion into the caskcavity occupied by the SNF, and operable to control and mitigate highpressure surge events experienced internally within the wet cask such asthose occurring under various postulated accidents and abnormaloperating conditions previously described herein. Advantageously, thisprotects the structural integrity of the cask from such high pressurizeexcursions which may be caused by external factors (such as fire ordegradation of the heat rejection process from the external surface ofthe cask) or a massive liberation of the gases encapsulated in the fuelrods due to degradation of this metal zirconia cladding described above.

Each pressure surge capacitor may be a fully welded and hermeticallysealed vessel (no credible path for leakage in or out) in one embodimentcomprising at least one rupture disk which seals an internal vacuumchamber inside the capacitor. Each rupture disk is designed andconstructed to burst at a predesigned/predetermined pressure level orcondition occurring inside the cask cavity external to the pressuresurge capacitor. This allows the excess cask pressure occurring during ahigh pressure excursion to bleed into capacitor, thereby returning thepressure inside the cask to acceptable levels. The vacuum cavity insideeach pressure surge capacitor is evacuated to sub-atmospheric (i.e.negative pressure) conditions to the greatest extend practicable. Thepressure surge capacitors may have an elongated tubular configuration insome embodiments.

The wet cask with hermetically sealable cavity may be used for variousapplications associated with operation of a nuclear reactor such as in anuclear power generation facility. For example, in one non-limitingapplication, the wet cask may be used to transfer spent nuclear fuelassemblies in a continuously submerged stated in the cask between spentfuel pools. The fuel assemblies may be loaded into the wet cask in afirst pool underwater, the cask may be lifted out of the first pool andtransported to and positioned in a second pool. Radiation blocking isachieved by maintaining the fuel assemblies in the water-impounded caskeven during transport.

Although the cavity of the cask may be configured and have appurtenancesdesigned to hold SNF assemblies in some embodiments, any type or form ofhigh level radioactive nuclear waste or irradiated materials may bestored in a submerged stated in the inventory of water held by the cask.Such high level radioactive waste materials may be collectively referredto as “radioactive nuclear waste.”

In one aspect, a storage system for radioactive nuclear waste comprises:a longitudinal axis; a cask comprising a hermetically sealable internalcavity configured to hold an inventory of water sufficient to submergethe nuclear waste therein; and a pressure surge capacitor disposed inthe cask, the pressure surge capacitor comprising a vacuum cavityevacuated to sub-atmospheric conditions; wherein the pressure surgecapacitor is configured to suppress a pressure surge in the internalcavity of the cask.

In another aspect, a cask with overpressurization protection for storingnuclear waste fuel comprises: a longitudinal axis; a cask bodycomprising a removable lid assembly, a base, and a circumferential wallincluding radiation shielding, the cask body forming a hermeticallysealed internal cavity configured for holding spent nuclear fuelsubmerged in an inventory of water; a pressure surge capacitor disposedin the cask, the pressure surge capacitor comprising a vacuum cavityevacuated to sub-atmospheric conditions; and the pressure surgecapacitor further comprising at least one rupture disk constructed toburst at a predetermined pressure level inside the cask associated witha cask overpressurization condition; wherein the rupture disk when burstallows a portion of the water to fill the vacuum chamber to reducepressure inside the cask.

In another aspect, a method for controlling pressure in a wet nuclearwaste storage system comprises: providing a cask comprising a sealableinternal cavity configured for storing nuclear waste; positioning apressure surge capacitor in the cask, the pressure surge capacitorcomprising a vacuum cavity evacuated to sub-atmospheric conditions andin fluid communication with the internal cavity; filling the cask withwater; submerging the nuclear waste in the water; and sealing a lidassembly to the cask to hermetically seal the internal cavity; whereinthe pressure surge capacitor is configured to suppress a pressure surgein the internal cavity of the cask. The method may further compriseafter the sealing step, steps of: increasing the pressure inside thecask to exceed a predetermined burst pressure of a rupture disk of thepressure surge capacitor; bursting the rupture disk; and admitting aportion of the water into the pressure surge capacitor which reduces thepressure inside the cask.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein likeelements are labeled similarly and in which:

FIG. 1 is top perspective view of a pressure vessel in the form of anunventilated hermetically sealable wet cask for storing and transportingradioactive nuclear waste such as SNF according to the presentdisclosure;

FIG. 2 is a bottom perspective view thereof;

FIG. 3 is a top exploded perspective view thereof;

FIG. 4 is a bottom exploded perspective view thereof;

FIG. 5 is a first side view thereof;

FIG. 6 is a second side view thereof;

FIG. 7 is a top view thereof;

FIG. 8 is a bottom view thereof;

FIG. 9 is a transverse cross sectional view taken from FIG. 6 throughthe lid assembly of the cask;

FIG. 10 is a perspective view of a spend nuclear fuel (SNF) assembly;

FIG. 11 is a longitudinal cross sectional view of the cask;

FIG. 12 is an enlarged detail from FIG. 11 ;

FIG. 13 is a perspective view of a pressure surge capacitor of the cask;

FIG. 14 is a side view thereof;

FIG. 15 is an end view thereof;

FIG. 16 is a side cross sectional view thereof;

FIG. 17 is an exploded end perspective view thereof;

FIG. 18 is an enlarged detail of the cask taken from FIG. 1 ;

FIG. 19 is a transverse cross sectional view of the cask taken from FIG.6 ;

FIG. 20 is a cross-sectional perspective view of the cask showing thepressure surge capacitor in a first mounting location in the cask;

FIG. 21 is a transverse cross-sectional of the cask showing the pressuresurge capacitor in a second mounting location in the cask; and

FIG. 22 is a transverse cross-sectional of the upper end of the caskshowing the pressure surge capacitor in a third mounting location in thecask.

All drawings are schematic and not necessarily to scale. Features shownnumbered in certain figures which may appear un-numbered in otherfigures are the same features unless noted otherwise herein. A generalreference herein to a figure by a whole number which may include relatedfigures sharing the same whole number but with different alphabeticalsuffixes shall be construed as a reference to all of those figuresunless expressly noted otherwise.

DETAILED DESCRIPTION

The features and benefits of the invention are illustrated and describedherein by reference to non-limiting exemplary (“example”) embodiments.This description of exemplary embodiments is intended to be read inconnection with the accompanying drawings, which are to be consideredpart of the entire written description. Accordingly, the disclosureexpressly should not be limited to such exemplary embodimentsillustrating some possible non-limiting combination of features that mayexist alone or in other combinations of features.

In the description of embodiments disclosed herein, any reference todirection or orientation is merely intended for convenience ofdescription and is not intended in any way to limit the scope of thepresent invention. Relative terms such as “lower,” “upper,”“horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and“bottom” as well as derivatives thereof (e.g., “horizontally,”“downwardly,” “upwardly,” etc.) should be construed to refer to theorientation as then described or as shown in the drawing underdiscussion. These relative terms are for convenience of description onlyand do not require that the apparatus be constructed or operated in aparticular orientation. Terms such as “attached,” “affixed,”“connected,” “coupled,” “interconnected,” and similar refer to arelationship wherein structures are secured or attached to one anothereither directly or indirectly through intervening structures, as well asboth movable or rigid attachments or relationships, unless expresslydescribed otherwise.

As used throughout, any ranges disclosed herein are used as shorthandfor describing each and every value that is within the range. Any valuewithin the range can be selected as the terminus of the range. Inaddition, any references cited herein are hereby incorporated byreference in their entireties. In the event of a conflict in adefinition in the present disclosure and that of a cited reference, thepresent disclosure controls.

The terms “seal weld or welding” as may be used herein shall beconstrued according to its conventional meaning in the art to be acontinuous weld which forms a gas-tight hermetically sealed jointbetween the parts joined by the weld. The term “sealed” as may be usedherein shall be construed to mean a gas-tight hermetic seal.

FIGS. 1-22 show various aspects and features of the wet cask system forstoring and transporting radioactive nuclear waste such as spent nuclearfuel (SNF) according to the present disclosure. The wet cask systemadvantageously overcomes the shortcomings of insert gas “dry casks”previously described herein. The relatively high conductivity of waterkeeps the fuel much cooler than does the inert gas medium in the drycask; an advantage that is highly desirable from the standpoint ofmaintaining a low pressure inside the nuclear fuel rods. The fuel rodsare elongated and thin-walled zirconium metal tubes (also called fuelcladding) containing the fuel pellets of fissionable material (e.g.uranium ceramic pellets) and an inert fill gas mixed with theradioactive gases in the tubes produced in the reactor core. Thehigh-pressure attendant to high temperature (pursuant to the classicalGas law) causes a high membrane stress in the fuel cladding which isknown to cause cladding's failure and release of its gaseous contentsinto the cask's fuel storage cavity. Preventing these harmful gases fromescaping into the environment is a key mission of a cask. The cask istherefore designed to withstand a rise in its internal pressure causedby failure of the fuel cladding. Assuming that a significant quantity ofgas release could occur during a cask's operation, designing itspressure retention capability with ample structural margin isaccordingly a mandatory requirement in virtually all regulatoryjurisdictions.

In a wet cask in which the spent nuclear fuel (SNF) assemblies areimmersed in water, the small free volume above the water and SNF (e.g.headspace) is occupied by water vapor. In case of a regulatorypostulated fire event, heating the captive volume of water in the wetcask can raise the vapor pressure in the headspace or an accidentleading to massive fuel rod failures inside the cask can release largequantities of the fuel rod gas into the cask cavity. Generation ofhydrogen and oxygen by radiolysis of water is another source of pressurebuild up, although this problem is largely overcome by the use ofpassively acting hydrogen recombiners or hydrogen “getters” placedinside the cask. The vulnerability of the wet cask to rapid pressurerise is further aggravated by the fact that, at high pressures, even asmall increase in the temperature causes a large bump in the saturationpressure.

To make wet casks safe and viable for storage/transport of high heatload used or spent nuclear waste fuel, a pressure control sub-system isdisclosed herein to protect the cask from a high internal pressure surgeunder the foregoing accident conditions.

With continuing reference in general to FIGS. 1-24 , the present wetcask system with integrated pressure control sub-system generallyincludes a leak-tight sealable cask 100 and at least one pressure surgecapacitor 200 operable to absorb a high pressure excursion occurringinternally within the cask. There are no provisions for circulatingambient cooling air through cask 100, which is distinguishable fromvertical ventilated type overpacks or casks well known in the industry.

Cask 100 may be a hermetically sealed, leak-tight pressure vesselcomprising a vertically elongated metallic cylindrical body 101 defininga vertical longitudinal axis LA passing through the vertical centerlineand geometric center of the body. The cask body generally includes (inprogression from top to bottom) lid assembly 110, annular top flange103, cylindrical circumferential wall 102, and circular base 104 atbottom. Circumferential wall 102 defines a circumferentially-extendingsidewall extending vertically between top flange 103 and base 104. Thetop and bottom ends of wall 102 may be fixedly coupled to the top flangeand bottom base via welded connections such as one or morecircumferentially continuous seal welds for each to permanently join thecomponents together. Top flange 103 and base 104 may be forged steelstructures in one embodiment for added mechanical strength in oneembodiment.

In some embodiments, the external surface of the cask circumferentialwall 102 may optionally comprise a plurality of annular heat transferfins 118 extending circumferentially around the cask 100. The fins maybe arranged in longitudinally spaced apart manner on the cask and extendin a vertical array between top flange 103 and bottom base 104 as shown.Since the sealed cask is not cooled by introducing and flowing ambientcooling air internally through the cask, the fins help remove heatemitted by the decaying fuel in the SNF assemblies in the cask. In otherembodiments, the fins may be omitted.

The cask body 101 defines an internal cavity 105 which extendslongitudinally for a full height of the cask from base 104 at bottom tothe top end of circumferential wall 102. The cavity 105 is configured indimensioned to hold a plurality of spent nuclear fuel (SNF) assemblies119 (see, e.g. FIG. 10 ). Cavity 105 is hermetically sealed when lidassembly 110 is removably coupled to the cask body. The fuel assembliesmay be insertably contained in a fuel basket 115 is disposed in cavity105 and seated on the bottom base 104. This design obviates the need fora typical unshielded fuel canister used with some casks. The presentcask 100 may instead be completely submerged directly into the spentfuel pool associated with the reactor for loading individual fuelassemblies into the basket while the assemblies remain immersed underwater for radiation containment.

The fuel basket 115 is a honeycomb prismatic structure comprising anarray of vertically-extending openings forming a plurality of verticallongitudinally-extending fuel assembly storage cells 116. Each cell isconfigured in cross-sectional area and shape to hold a single U.S. stylenuclear fuel assembly 119 (see, e.g. FIG. 10 ) having a rectangularcuboid configuration, which in turn contains a multitude of spentnuclear fuel rods 119 a previously described herein (or otherradioactive nuclear waste). The cells 116 may each have generally squarecross-sectional shape as shown which is complementary configured to thecross sectional shape of the fuel assembly. Such fuel assemblies and theforegoing fuel basket structure are well known in the industry. The fuelbasket may be formed in various embodiments by a plurality ofinterlocked and orthogonally arranged slotted plates built up to aselected height in vertically stacked tiers of plates. Examples ofslotted plate basket constructions are disclosed in commonly-owned U.S.patent application Ser. No. 17/115,005, which is incorporated herein byreference. Other constructions of fuel baskets such as multiplelaterally adjacent vertically extending tubes or other structures to thecanister baseplate may be used and others used in the art may be used.In addition, the fuel assembly cells 116 in some constructions may havea hexagonal cross-sectional shape to accommodate hexagonal fuelassemblies commonly used in Russia. The fuel basket construction howeveris not limiting of the present invention.

As best shown in FIG. 23 , the polygonal shaped structure of thecomplete fuel basket 115 structure fitted inside the cylindricalinternal cavity 105 of cask 100 leaves a plurality of unused peripheralareas or regions 117 between the cask circumferential wall 102 andbasket. These regions have a par-polygonal shape comprising one outernon-polygonal side formed by an arcuate portion of the cask wall 102 andremaining polygonal inner sides of linear shape formed by parts of thefuel basket 115. Peripheral regions 117 are dead zones serving typicallyno function and considered wasted space. Accordingly, such peripheralregions are generally kept to a minimum as much as practical. Thepresent cask pressure control sub-system however advantageously makesuse of these dead zones, as further described herein.

Referring initially in general to FIGS. 1-14 and 20-22 , wet cask 100 isa heavy radiation shielded nuclear waste fuel storage and transportvessel having a composite wall construction operable to ameliorate thegamma and neutron radiation emitted by the SNF fuel assemblies containedtherein to safe levels outside the cask. Circumferential wall 102 of thecask comprises in progression from inside to outside inner shell 120adjacent to cask cavity 105, intermediate shell 121, and outer shell 122(see, e.g. FIGS. 11, 12, and 21 ). Inner shell 120 may be formed ofthick steel. Shells 120-122 are coaxially aligned around longitudinalaxis LA and radially spaced apart to permit radiation shieldingmaterials to be located in the annular gaps or spaces formed between theshells. Gamma shielding material 123 is disposed in inner annular space125 between inner shell 120 and intermediate shell 121. Any suitablegamma shielding material may be used, including lead as shown, concrete,or others. In one embodiment, gamma shielding material 123 may beprovided in the form of longitudinally-elongated arcuately curved blockseach extending for a majority of the height of the cask 100 at leastcovering a portion of the height of the fuel basket which contains theSNF fuel assemblies 119. A plurality of such blocks are arrangedcircumferentially around the cask encircling internal cavity 105. Thegamma shielding blocks may be separated by conductive inner radial ribs127 welded between and to inner shell 120 and intermediate shell 121(see, e.g. FIG. 21 ). Ribs 127 may be made of steel in one embodiment. Aplurality or array of circumferentially spaced apart ribs 127 encirclethe inner shell 120. The ribs 127 form longitudinally-extending pocketswhich receive and organize the blocks of the gamma shielding material.Notably, ribs 127 further act as thermally conductive elements whichdraw the heat emitted by the SNF assemblies 119 outwards towards theouter shell 122 and heat transfer fins 118 since there is no ambientventilation air circulated through this unventilated cask. Ribs 127further provide structural reinforcement for the cask and maintain theannular space 125 between the shells 120, 121.

Boron-containing neutron shielding material 124 is disposed in outerannular space 126 between outer shell 122 and intermediate shell 121.Any suitable neutron shielding material containing boron may be used,such as for example without limitation Holtite™ from HoltecInternational of Camden, Jersey. Other boron-containing materialshowever may be used. In one embodiment, neutron shielding material 124may be provided in the form of longitudinally-elongated bars eachextending for a majority of the height of the cask 100 at least coveringa portion of the height of the fuel basket which contains the SNF fuelassemblies. A plurality of such bars are arranged circumferentiallyaround the cask encircling internal cavity 105 which holds the SNF fuelassemblies 119. The neutron shielding bars may be similarly separated byan outer second plurality or array of conductive outer radial ribs 128welded between and to outer shell 122 and intermediate shell 121 (see,e.g. FIG. 21 ). The ribs form longitudinally-extending pockets whichreceive and organize the bars of the neutron shielding material (e.g.Holtite™), as well as providing the same heat transfer function andstructural reinforcement for the cask as ribs 127 described above inaddition to maintaining the annular space 126 between the shells 121,122. Outer radial ribs 128 may be formed of copper in one embodiment tomaximize heat transfer between the intermediate shell 121 and outershell 122.

In some embodiments, the forgoing inner radial ribs 127 and/or the outerradial ribs 128 may each be formed as integral parts of an annular orring-shaped monolithic casting. Each of the castings may then be fittedbetween the shells 120-122 in the circumferential wall 102 of the cask100 in their respective positions described above. In one embodiment,the outer radial rib 128 casting may be made of copper to maximize heattransfer. Inner radial rib 127 casting may be formed of steel in someembodiments if used. Alternatively, either of the inner or outer radialribs 127, 128 may be welded directly to their respective shells whichthey bridge.

Lid assembly 110 may comprise a lower inner lid 111 and upper outer lid112 stacked thereon (best shown in FIGS. 3-4 and 12 ). Inner lid 111 isconfigured for partial insertion into cask cavity 105 and terminatesproximate to the top of the fuel basket 115. A lower portion of the lid111 therefore has a smaller diameter than the inside diameter of thecask defined by inner shell 120. An upper portion of inner lid 111 has alarger diameter radially protruding annular flange 111 a seated on amating step-shaped annular shoulder 113 a on the top of the cask definedby cask top flange 103. Lid 111 may include a centrally located liftinglug 113 on top configured to be grasped by a grappling assembly of ahoist or crane for lifting the lid into place on the cask 100. Liftinglug 113 may be disk-shaped in one embodiment. Lug 113 is received andnested in a downwardly open complementary configured circular recess 114formed on the bottom of the outer lid 112. Outer lid 112 has a largerdiameter than the inner lid and comprises a radially protruding annularflange 112 a seated on a mating step-shaped annular shoulder 114 a onthe top of the cask also defined by cask top flange 103. Outer lid 112is bolted to top flange 103 of cask 100 by a circular array of closurebolts 112 b, thereby trapping the inner lid 111 onto the cask.

Lid assembly 110 further comprises a plurality of annular seals 150compressed between the cask body (e.g. top flange 103) and each of theinner and outer lids 111, 112 (best shown in FIG. 12 ). When lidassembly 110 is coupled to cask 100 (e.g. bolted), a hermetically sealedleak-tight cask internal cavity 105 and pressure vessel is created whichis fluidly isolated from the ambient environment.

Cask 100 may further include a plurality of radially protruding liftinglugs 130 for maneuvering the cask such as lifting into and out of thespent fuel pool during the process of loading SNF assemblies 119 intothe cask fuel basket 115. At least one bottom drain assembly 160 may beprovided which is openable/sealable to drain the inventory of water inthe cask in which the fuel assemblies are submerged (FIG. 11 ). Drainassembly 160 may be formed in base 104 in some embodiments. A topopenable/sealable port 161 which is fluidly coupled to cask internalcavity 105 via a duct as shown in FIG. 12 . Port 161 may be used forvarious purposes, including for example without limitation for testingthe conditions inside the cask, or optionally to convert the cask 100 tolong dry storage by circulating an inert gas (e.g. helium) through thecask to dry cavity 105 in conjunction with the bottom drain assembly 160for establishing a gas flow path therethrough. Inert gas cask dryingsystems are well known in the art without further elaboration. Top port161 may be a gas inlet and bottom drain assembly 160 may be a gasoutlet, or vice versa. Top port 161 may be formed in top flange 103 insome embodiments.

As previously described herein, wet cask 100 is a water-impounded caskin which the fuel assemblies 119 are immersed under water. The water Whas a surface level sufficient to at least fully cover the fuelassemblies. An exemplary surface level of water W is represented in FIG.12 by the dashed line.

The pressure control sub-system comprising pressure surge capacitor 200operable to absorb a high pressure excursion occurring internally withinthe cask will now be further described. FIGS. 13-17 show the pressuresurge capacitor in isolation.

Referring initially to FIGS. 13-17 and 21-22 , pressure surge capacitor200 has a longitudinally elongated cylindrical tubular body defining avertical centerline and comprising a top end 201, bottom end 202, andcylindrical sidewall shell 203 extending therebetween and defining aninternal pressurizable vacuum space or chamber 204 having a volume V1.The terminal end portions of the capacitor 200 define end caps 206having a thickness measured parallel to the vertical centerline Vc whichis substantially greater (e.g. 3 times or more) than the wall thicknessof the sidewall shell 203 (measured transversely to centerline Vc).

As shown in FIGS. 11 and 22 , the pressure surge capacitor 200 is alongitudinally elongated pressure vessel having a greater longitudinallylength than its diameter. Capacitor may have a height at leastcoextensive with the height of the fuel basket 115 in some embodiments.Capacitor 200 therefore has a height which extends for a substantialmajority of the height of the internal cavity 105 of cask 100 fromproximate to the bottom of lid assembly 110 to base 104 of the cask. Inone embodiment, pressure surge capacitor 200 is positioned and locatedadjacent to inner shell 120 of the cask in its internal cavity 105, suchas in one of the larger peripheral regions 117 inside cask 100 lyingbetween the fuel basket 115 and inner shell 120 (see also FIG. 19 ).This otherwise dead space too small to accommodate a full rectangularSNF fuel assembly is advantageously not wasted and advantageously usedin a cask overpressurization function. Although one pressure surgecapacitor 200 is shown, other embodiments may place multiple capacitors200 in a similar manner in peripheral regions 117 for added caskpressure surge protection as needed.

A flow inlet opening 205 is formed through at least one end 201 or 202of pressure surge capacitor 200 (e.g. end caps 206), and in someembodiments through both ends at top and bottom as shown. Inlet opening205 is in fluid communication with vacuum chamber 204 of pressure surgecapacitor 200 for selectively admitting high pressure water held insidecask cavity 105 during a cask n internal pressure excursion (increase).In one embodiment, inlet opening 205 may be circular in transverse crosssection and comprises a larger diameter outer portion 209 and smalldiameter inner portion 210. A step-shaped annular shoulder 211 is formedtherebetween (best shown in FIGS. 16 and 17 ).

Each inlet opening 205 is fitted with a pressure relief device 220comprising a circular metallic rupture disk 221 and annular diskretaining ring 222. Retainer ring 222 includes a central opening 223which allows pressurized water to flow through the inlet opening 205into the pressurizable vacuum chamber 204 of the tubular pressure surgecapacitor when the rupture disk bursts. Rupture disk 221 is designed andconstructed with predetermined burst pressure selected to protect thecask 100 and fuel assemblies 119 therein from a potentially damaginghigh pressure condition previously described herein internal to the caskcaused by degradation and/or failure of the fuel rod cladding. Thepredetermined burst level is set taking into consideration thedifferential between the sub-atmospheric vacuum condition inside thepressure surge capacitor 200 and the pressure outside the capacitorinside the cask cavity 105. Any suitable type of metal rupture disk maybe used, including without limitation a reverse buckling design as shownherein (in which the convex side of the rupture disk faces the highpressure source) or a forward-acting disk design (in which the concaveside of the disks faces the high pressure source).

During assembly of each pressure relief device 220, one rupture disk 221is positioned and seated on an outward facing disk seating surface 224formed at the innermost end of outer portion 209 of flow inlet opening205. One retaining ring 222 is then positioned over the rupture disk andcoupled to the end cap 206 of pressure surge capacitor 200 such as viawelding, threaded connection, or other. This traps the rupture disk 221between the retaining ring and seating surface 224. The dome shapedcentral portion of the rupture disk protrudes outwards into andpartially enters the central opening 223 of retaining ring 222 where itis exposed to the internal pressure of cask 100 inside cavity 105.

Each pressure surge capacitor 200 is then evacuated to as deep a vacuum(negative pressure) as practicable. A vacuum port 230 may be formed insidewall shell (or alternatively the end caps 206) for evacuating thevacuum chamber 204 of the capacitor. A valve 231 may be removablycoupled to the port 230 for drawing the vacuum via an external vacuumpump 232 (valve and pump shown schematically in FIG. 18 ). Any suitabletype of valve may be used which is configured with a suitable endfitting configuration for detachable coupling to a hose or other flowconduit line fluidly connected to the vacuum pump.

The evacuated pressure surge capacitor(s) 200 are now ready fordeployment and operation. Each pressure surge capacitor 200 provided(e.g. one or more) may be positioned inside cask cavity 105 in anavailable empty space such as open peripheral regions 117 (see, e.g.FIG. 19 ). Capacitors 200 may be loosely positioned in the cask, oralternatively may be fixedly attached to the outside walls of the fuelbasket 115 (such as via welding) before the fuel basket is installed incask 100. In the latter case, the capacitors may be evacuated before orafter welding to the fuel basket. Capacitors 200 are constructed viaselection of the type of metal used for the body and end caps (i.e.mechanical strength and other material properties), and associatedthicknesses to withstand the external pressure which the cask cavitywill exert from the elevated hydraulic pressure that would result undera condition of elevated temperature of the body of water inside cask100. The pressure surge capacitors are fully exposed to the temperatureand pressure conditions inside the cask cavity 105.

In operation, if an overpressurization condition should occur in caskcavity 105 which exceeds the pre-designed and predetermined burstpressure of the rupture disk 221, the disk will burst allowing theexcess pressure to bleed into the evacuated vacuum chamber 204 of thecapacitors 200 (see dashed water inflow arrows in FIG. 18 ). Theinternal cask pressure will attempt to equilibrate inside and outsidethe pressure surge capacitors in the cask cavity 105 to thereby lowerthe internal cask pressure to a stable acceptable pressure level,thereby ameliorating the high pressure excursion condition. Althoughonly a single pressure surge capacitor 200 is shown for clarity ofdepiction in the figures, it will be appreciated that other embodimentswill include any suitable number of pressure surge capacitors as neededto provide the surge capacity necessary to compensate for and amelioratethe postulated cask overpressurization conditions that could possiblyoccur during storage of the SNF in sealed wet cask 100.

A method for controlling pressure in a sealed cask 100 using pressuresurge capacitors 200 will now be briefly summarized. The methodgenerally comprises providing an unventilated cask 100 comprising asealable internal cavity 105 configured for storing nuclear waste suchas spent nuclear fuel assemblies 119; positioning a pressure surgecapacitor 200 in the cask, the pressure surge capacitor comprising avacuum cavity 204 evacuated to sub-atmospheric conditions and in fluidcommunication with the internal cavity; filling the cask with water;submerging the nuclear waste in the water; and sealing a lid assembly110 to the cask to hermetically seal the internal cavity; wherein thepressure surge capacitor is configured to suppress a pressure surge inthe internal cavity of the cask. The method may further include afterthe sealing step, steps of: increasing the pressure inside the cask toexceed a predetermined burst pressure of a rupture disk 221 of thepressure surge capacitor 200; and admitting a portion of the water intothe pressure surge capacitor which reduces the pressure inside the cask.The pressure surge capacitor therefore advantageously operates torelieve the cask pressure and ameliorate the high pressure increaseexcursion.

In some embodiments, the filling step includes lowering the cask 100into a first spent fuel pool 250 below a water surface S thereof(schematically shown in FIG. 11 ). The method may further include afterthe sealing step, steps of: lifting the cask out of the first spent fuelpool; and transporting the cask to a second spent fuel pool. The caskmay be lowered into the second fuel pool for either loading additionalSNF assemblies 119 into the cask, or unloading the spent fuel assembliesinto the second fuel pool such as into cavities of a SNF storage racksuch as those disclosed in commonly-owned U.S. Pat. No. 10,847,274,which is incorporated herein by reference in its entirety.

Variations in the foregoing steps of the method, and additional steps,may be used.

It bears noting pressure surge capacitor 200 is shown having acylindrical configuration, in other embodiments the capacitor may have abody shaped other than cylindrical with circular transverse crosssection, such as any suitable non-polygonal or polygonal configuration.The shape of the pressure surge capacitor does not limit the invention.

FIG. 21 shows an alternative embodiment of the pressure controlsub-system of the cask 100 in which one or more pressure surgecapacitors 200 a are incorporated and embedded in the circumferentialwalls 102 of the main cylindrical body of the cask. Inner shell 120includes one or more separate flow apertures 151 fluidly coupled betweeneach rupture disk 221 provided for the capacitor 200 a and the internalcavity 105 of the cask 100. The vacuum chamber 204 of the capacitor istherefore fluidly connected to the cavity 105 through the rupturedisk(s) 221 previously described herein and function in the same mannerto protect the cask from internal high pressure surges/excursions. Aplurality of embedded pressure surge capacitors 200 a may be provided.

FIG. 22 shows a second alternative embodiment of the pressure controlsub-system of the cask 100 in which one or more pressure surgecapacitors 200 b are incorporated in the lid assembly 110 of the cask100. In this illustrated embodiment, a centrally located pressure surgecapacitor 200 b is fixedly coupled to the bottom surface of inner lid111 (such as via welding). This locates capacitor 200 b in the headspacebetween the bottom surface of inner lid 111 and the top edges of thefuel basket within the internal cavity 105 of the cask 100. Capacitor200 b may have a cylindrical body similar to capacitor 200 previouslydescribed herein and includes at least one pressure relief device 220(i.e. rupture disk 221 and retaining ring 222) in a bottom surface ofthe lid-mounted capacitor 200 b. In this embodiment, the diameter ofcapacitor 200 b may be larger than its longitudinal height. In someembodiments, an array comprised of multiple lid-mounted pressure surgecapacitors 200 b may instead be provided.

It bears noting that alternative pressure surge capacitors 200 a and 200b may be provided instead of pressure surge capacitors 200 previouslydescribed herein which are located directly in the cask fuel storageinternal cavity 105, or alternatively in addition thereto if addedpressure surge amelioration capacity is needed.

While the foregoing description and drawings represent some examplesystems, it will be understood that various additions, modifications andsubstitutions may be made therein without departing from the spirit andscope and range of equivalents of the accompanying claims. Inparticular, it will be clear to those skilled in the art that thepresent invention may be embodied in other forms, structures,arrangements, proportions, sizes, and with other elements, materials,and components, without departing from the spirit or essentialcharacteristics thereof. In addition, numerous variations in themethods/processes described herein may be made. One skilled in the artwill further appreciate that the invention may be used with manymodifications of structure, arrangement, proportions, sizes, materials,and components and otherwise, used in the practice of the invention,which are particularly adapted to specific environments and operativerequirements without departing from the principles of the presentinvention. The presently disclosed embodiments are therefore to beconsidered in all respects as illustrative and not restrictive, thescope of the invention being defined by the appended claims andequivalents thereof, and not limited to the foregoing description orembodiments. Rather, the appended claims should be construed broadly, toinclude other variants and embodiments of the invention, which may bemade by those skilled in the art without departing from the scope andrange of equivalents of the invention.

What is claimed is:
 1. A method for controlling pressure in a wetnuclear waste storage system comprising: providing a cask comprising asealable internal cavity configured for storing nuclear waste;positioning a pressure surge capacitor in the cask, the pressure surgecapacitor comprising a vacuum cavity evacuated to sub-atmosphericconditions and in fluid communication with the internal cavity; fillingthe cask with water; submerging the nuclear waste in the water; andsealing a lid assembly to the cask to hermetically seal the internalcavity; wherein the pressure surge capacitor is configured to suppress apressure surge in the internal cavity of the cask.
 2. The methodaccording to claim 1, further comprising after the sealing step, stepsof: increasing the pressure inside the cask to exceed a predeterminedburst pressure of a rupture disk of the pressure surge capacitor;bursting the rupture disk; and admitting a portion of the water into thepressure surge capacitor which reduces the pressure inside the cask. 3.The method according to claim 1, wherein the positioning step includeslocating the pressure surge capacitor in a peripheral region of theinternal cavity of the cask.
 4. The method according to claim 1, whereinthe submerging step further includes inserting at least one spentnuclear fuel assembly into a fuel basket in the internal cavity of thecask.
 5. The method according to claim 1, wherein the positioning stepincludes affixing the pressure surge capacitor on an underside of thelid assembly.
 6. The method according to claim 1, wherein thepositioning step includes installing the pressure surge capacitor withina circumferential wall of the cask.
 7. The method according to claim 1,wherein the filling step includes lowering the cask into a first spentfuel pool.
 8. The method according to claim 7, further comprising afterthe sealing step, steps of: lifting the cask out of the first spent fuelpool; and transporting the cask to a second spent fuel pool.
 9. Themethod according to claim 2, wherein the positioning step includeslocating the pressure surge capacitor in a peripheral region of theinternal cavity of the cask.
 10. The method according to claim 2,wherein the positioning step includes affixing the pressure surgecapacitor on an underside of the lid assembly.
 11. The method accordingto claim 2, wherein the positioning step includes installing thepressure surge capacitor within a circumferential wall of the cask.